Intra-aortic balloon pump therapy is frequently prescribed for patients who have suffered a heart attack or some other form of heart failure. In such therapy, a thin balloon is inserted through an artery into the aorta. The balloon is connected through a series of thin tubes to a complex apparatus which causes the balloon to inflate and deflate in time with the patient's heart beat, thereby assuming some of the load on the heart during the patient's recovery period.
Included in the complex inflation/deflation apparatus is a compressor which acts on a balloon driving mechanism to supply positive pressure for expanding the balloon during an inflation cycle and negative pressure for contracting the balloon during a deflation cycle. Such compressors conventionally include a pair of pistons which reciprocate out of phase with one another so as to supply the positive and negative pressures alternately. Thus, one reciprocating piston draws in outside air, compresses it and expels the air toward the balloon driving mechanism. The other piston draws air out from the balloon driving mechanism and expels it from the compressor to the atmosphere.
Each piston typically reciprocates in a cylinder having a valve plate which is provided with two reed valves, one controlling the flow of air into the cylinder and one controlling the flow of air out from the cylinder. During the intake stroke of a piston, the intake reed valve permits air to be drawn into the cylinder through the cylinder inlet, while the discharge reed valve prevents air from being drawn in through the cylinder outlet. Similarly, during the exhaust stroke of the piston, the discharge reed valve permits air to be expelled from the cylinder through the cylinder outlet, while the intake reed valve prevents air from being expelled through the cylinder inlet. The air flowing through the reed valves generates a sound much like the sound generated by the reed in a wind instrument, which sound is continually repeated or pulsed as a result of the reciprocating motion of the pistons. The turbulent flow of the air as it travels at high velocity into and out from the cylinders also generates acoustic noise in a pulsating fashion. As a result of the irritating pulsing noise they generate, the use of these compressors in hospital settings is undesirable.
Efforts have been made to attenuate the pulsing sound emitted by these compressors. Heretofore, such efforts typically have encompassed providing the inlet and outlet of the compressor with a dissipative muffler consisting of an enclosed chamber filled with a porous material. While such mufflers effectively attenuate very high frequencies, they have little affect on lower frequency sounds.
The use of non-dissipative mufflers for reducing sounds within a specific frequency range has long been recognized. One such muffler, a Helmholtz resonator, consists of a large gas-filled chamber having a small outlet. The resonator can be tuned to maximize the amount of attenuation by adjusting the length and diameter of the outlet with respect to the size of the chamber. Typically, rather large chambers are required in order to achieve effective low frequency attenuation. Hence, while these types of resonators are more effective than dissipative mufflers in attenuating pulsing sounds in the audible frequencies, they have received little consideration in connection with medical compressors where the ultimate size of the apparatus is a significant concern.
There therefore exists a need for improved sound attenuation in pneumatic compressors, and particularly those used to drive medical devices in a hospital setting. Preferably, these improvements can be achieved without significantly affecting the overall size of these compressors or the cost of manufacturing same.